A shape of a work is complicated in recent years in a machining of a machine tool, and the machining on the basis of a five-axis simultaneous control is going to be carried out for the purpose of shortening a machining time and improving a machining accuracy. In the case that the number of the machining axes which are controlled simultaneously is increased, an error called generally a geometric error, for example, an inclination between two axes and a deviation of a center position in a pivot shaft becomes a subject of discussion, and there is generated needs for measuring a machine error of a finished machine tool at a higher accuracy.
FIG. 8 is a view schematically showing an example which measures a deviation of a center position in a pivot shaft of a table 42 in a machining center by using a five-axis measuring method. A reference sphere 2 is fixed to an outer peripheral portion of the table 42, and a touch probe 1 is installed to a tool shaft 41 which is provided in a tool post 43. Further, a central coordinate of the reference sphere 2 is determined by bringing the touch probe 1 into contact with the reference sphere 2 from five directions at positions where the table 42 is rotated at every predetermined angle (for example, five degrees).
In the machining center, a rotating movement of the table 42 is generally generated by a C direction (around a Z-axis), and an upper surface of the table 42 is an X-Y plane. Five directions in this case are constituted by −Y, +Y, −X, +X and +Z directions (refer to FIG. 8).
In the case that the touch probe 1 is installed to the tool post 43 in such a manner that a lever rod 12 extends in a direction of the Z-axis, a tip sphere 11 of the touch probe can be brought into contact with the positions in five directions shown in FIG. 9 whichever position the table 42 rotates to.
A measurement program for measuring a central coordinate of a reference sphere by bringing a touch probe into contact with a reference sphere according to the procedure mentioned above and arithmetically operating a machine error on the basis of the measured value has been provided, for example, by a manufacture of the touch probe, and has been conventionally used for measuring a machine accuracy of a machining center.